Flexible polyurethane foams are known to have a very wide variety of applications and are used in particular in the automotive industry and in the furniture industry. The quality of the flexible polyurethane foams normally used for cushioning seats and armrests is determined inter alia by their density. Regardless of the type of raw materials (that is, isocyanates or polyols) used for the production of molded foams, a certain density level has proved to be effective and practical for the various applications and associated demands involved.
It has previously been possible to produce moldings of varying density for the same foam formulation only by using physical blowing agents. In particular, the easily handled chlorofluorocarbons ("CFC's" and "HCFC's"), especially fluorotrichloromethane ("CFC 11"), have acquired a prominent position in this regard. The use of reduced pressures during the foaming process to reduce density has not previously been successful because of the elaborate techniques involved. Increasing the temperature of the raw materials while not otherwise changing the foaming parameters has not previously appeared to be a suitable way to obtain low density because gas volume calculations indicated the possibility of only a minimal reduction in density. That is, model calculations based on the ideal gas equation produce an estimated increase in density of approximately ##EQU1## for an increase in the temperature of the raw material of 15.degree. C. This calculation is based on the assumption that the mean reaction temperature is 80.degree. C. It has, however, now surprisingly been found that an increase in the temperature of the raw material of 15.degree. C., for example, from 25.degree. C. to 40.degree. C., produces a reduction in density of more than 10%. This reduction cannot be explained by the temperature-induced increase in volume of the carbon dioxide formed.
The production of flexible foams is normally carried out at a raw material temperature in the range from 20.degree. C. to 30.degree. C. A temperature of 45.degree. C. for example has not previously been used because the CFC 11 used would be likely to evaporate or because the reactivity of the reactants would be drastically increased compared with their reactivity at room temperature. The process according to the invention, however, now enables CFC's and HCFC's to be dispensed with altogether. Accordingly, by using the method of this invention, both problems attributable to increases in temperature can be avoided.
The increase in reactivity brought about by the increased temperature of the raw material naturally leads to different flow behavior because the reaction mixture creams at an earlier stage. However, at least equivalent flow behavior can be obtained by modifying the catalysis, for example, by using blocked amines and/or by reducing the quantity of catalyst. The reduction in the initial viscosity even leads to tendentially better flow.
Accordingly, the present invention relates to a process in which a distinctly lower density of the flexible foam moldings formed is obtained solely by an increase in the raw material temperature.